International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
INFLUENCE OF PROTECTIVE COATING ON BOND STRENGTH
DEVELOPMENT OF STEEL WITH CONCRETE
Ms.B.Saritha*, Dr. M.S. Haji Sheik Mohammed**, Dr. R. Srinivasa Raghavan***
* Bharath university selaiyur Chennai.
*** Associate Professor, B.S.Abdur Rahman University, Chennai.
** Professor& Head, Department of Civil Engineering, B.S.Abdur Rahman University
ABSTRACT
This investigation presents the influence of protective coating on bond strength
development of steel with concrete. Pull-out test was conducted in the 1000 kN capacity
Universal Testing Machine as per procedure outlined in Indian Standards. The tested rebar
includes rusted (uncoated) rebar as available in the site, acid pickled rebar (Rust free rebar)
and cement polymer anticorrosive coated rebar obtained from a single source. Concrete of
M25 grade were used and totally 18 specimens were tested. The loads at the free end slip of
0.025mm, 0.25mm, loaded end slip of 0.25mm and ultimate pull out load were observed. The
load-versus slip behaviour was studied at the free end and loaded end using precision dial
gauges. The bond stress corresponding to the lowest load value of 0.025mm FE slip and
0.25mm LE slip was considered as the usable bond strength. The test results revealed that a
well defined correlation exists between Load at 0.025mm Free end slip and 0.25mm Loaded
end slip. It was found that presence of rust and cement polymer anticorrosive coating in the
steel concrete interface appreciably increases the bond strength of the order of 16%, 35%
respectively for 16mm diameter bars as compared to rust free rebar. For 20mm dia. bars,
marginal reduction in bond strength for rusted rebars and significant increase in bond
strength for coated bars as compared to uncoated bars were observed. It is concluded that
presence of rust influences in reduction / increase in bond strength depending on the nature
of rust at the interface in the initial ages. Application of cement polymer coating improves the
bond strength of the order of 30 – 35% due to excellent compatibility at the steel concrete
interface, satisfies the codal requirements of Indian Standards.
INTRODUCTION
The bond between rebar and concrete is essential for reliable performance of
reinforced concrete structures(8). The rebar concrete bond is due to various parameters such
as chemical adhesion, bearing action of concrete against deformations of rebar and frictional
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
resistance offered by concrete against movement. The introduction of coating at the steel
concrete interface will definitely modify the bond behaviour of steel with concrete. The
application of protective coating to rebar becomes an efficient and popular method of
protecting steel rebar from corrosion inside the concrete (7). The most widely used
commercial coating methods are Fusion Bonded Epoxy Coating, Galvanization, Cement
Polymer Composite coating and Inhibited Cement Slurry Coating. Nowadays a simple,
Cement Polymer Anticorrosive Coating was developed and used in various parts of South
India. It has been reported that epoxy coated bars offers less slip resistance and bond strength
as compared to uncoated bars. The bond splitting cracks are also initiated at lower loads for
epoxy coated bars(1). There is no adverse effect on bond strength for galvanized steel even
without chromate treatment where as epoxy coating reduces the bond strength of the order of
20% as compared to uncoated bars (6). The improved bond strength upon increase in coating
thickness was also reported by the researchers (8). It is necessary to investigate the influence
of cement polymer anticorrosive coating at the steel concrete interface on the bond strength
development.
EXPERIMENTAL INVESTIGATION
The mix design for M25 concrete was carried out as per American Concrete Institute
(ACI) method. The concrete material comprises of 20 mm downgraded jelly, natural sand
available from local source, 53 grade Ordinary Portland Cement obtained from single source
and potable water. The Mix ratio obtained is 1 : 1.6 : 3.2, w/c ratio 0.45. The steel rebar
comprises of heavily rusted 16 mm and 20 mm TMT rebar obtained from a single length. The
coating material is a commercially available Cement Polymer Anticorrosive solution CORROCON – CPAC.
The type of rebar used in the study includes rusted rebar (uncoated bar) as available in
the site, the rebar pickled in 20% HCL acid for rust removal (rust free rebar) and the rebar
coated with Cement polymer anticorrosive coating. To maintain uniform material and
chemical properties, all the three type of rebars were prepared from the single rebar length.
The coated bars were prepared by thoroughly cleaning the loose rust over rebar by steel wire
brush followed by application of one coat of Polymer cementious anticorrosive slurry ( Mix
proportion : 500 ml. Anticorrosive polymer solution with 1 kg. cement).
Pull out test was carried out as per IS 2770 – 1967 – Part I ( Methods of Testing Bond
in Reinforced Concrete) considering the modifications outlined in IS 1786 – 1985(3). The test
results were compared with IS 13620 – 1993(5). Three each in rusted rebars, rust free rebars
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
and coated rebars were tested. The diameter of the rebar varied is 16mm and 20mm. Totally
18 pull-out tests were conducted. Universal Testing Machine of 1000 kN capacity (Model :
UTES 100) with computer interface was used to carryout the test. Concrete cubes of size
150mm were cast with centrally embedded rebar provided upto 20mm from the bottom face
of the cube.The rebar extended over the top face of the cube upto sufficient length to
facilitate gripping on the bottom platen of the machine. To resist bursting stress during
testing, the concrete cube were provided with 6mm diameter helical reinforcement with
130mm diameter and 25mm pitch.
Figure 1 shows the arrangement of rebar inside the mould. The test was carried out as
per IS 2770 -1967(4). Figure 2 shows the Pull-out test in progress. The load – slip behaviour
was observed using sophisticated dial gauges at free end and loaded end. The load at the Free
end slip of 0.025mm, 0.25mm and loaded end slip of 0.25mm were found out. The ultimate
pull-out load was also observed.
RESULTS AND DISCUSSION
According to Indian standards the rebars used in the Reinforced cement concrete
should be free from rust. The Bond strength results of rust free rebars are compared with
rusted rebar, coated rebar and analysed. The lowest bond strength value at the measured free
end slip of 0.025mm and loaded end slip of 0.25mm was considered as the usable bond
strength. Figure 3(a) and (b) shows the Load versus slip behaviour of rust free, rusted and
coated rebars at the free end. It can be seen that at the 0.025mm slip level, the load values
observed for coated bar is appreciably higher than the rusted and rust free rebar. It is due to
the fact that at the initial stage of pull-out, the resistance offered is due to chemical adhesion
between cement concrete and steel. Since the coating material contains cement, the
simulataneous hydration of cement in the coating material and concrete improves the
adhesion appreciably as compared to rust free and rusted rebar. It can also be seen that the
bond strength at 0.025mm slip for rusted and rust free rebar is not following a standard
pattern irrespective of the diameter of rebar. It is due to the fact that presence of rust at the
interface may influence in increment or reduction in in bond strength at the initial stages
depending upon the nature and intensity of rust.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
Fig.1 Arrangement of Rebar inside
Fig. 2 Pull-Out Test in Progress
Fig. 3 Load versus Slip Behaviour of Rebars at the Free End
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
Figure 4(a) and (b) shows the Load versus slip behaviour of rust free, rusted and
coated rebars at the loaded end. It can be seen that the behaviour at the 0.25mm slip level is
almost similar to that of free end behaviour for both 16mm and 20mm rebar. The important
observation from the obtained behaviour is that the load values at 0.25mm LE slip is almost
similar to the 0.025mm FE slip values. The Indian standards gives more important to the
0.025mm FE slip and 0.25mm LE slip. Therefore the lowest bond strength obtained from
both the above condition is the usable bond strength. The ultimate pull out strength is the
other important parameter emphasised by the IS 2770 – 1967. Hence bond strength was also
anayled based on this values.
Table 1 shows the observation on Pull-out test for Rust free, Rusted and Coated
rebars. It can be observed that load values at 0.025 FE slip, 0.25mm LE slip were appreciably
higher for coated rebars as compared to Rust free and Rusted rebars irrespective of diameter
of rebar. Whereas ultimate strength obtained for coated bars is marginally less as compared to
rusted and rust free rebars. This may be due to the fact that the later stage bond resistance
contributed from bearing action of rib and frictional resistance is marginally affected for
coated rebars due to the existence of barrier layer for 200 micon thickness at the interface.
Table 2 shows the usable bond srength values for Rust free, Rusted and Coated rebars for
both 16mm and 20mm diameter rebars. It can be observed that there is an appreciable bond
strength improvement for Rusted and Coated rebars of the order of 16% and 35%
respectively as compared to Rust free rebars in case of 16mm diameter bars. For 20mm
diameter bars, appreciable reduction in bond strength of the order of 12% for Rusted rebars as
compared to Rust free rebar, whereas coated rebar improves the bond strength upto 29%. The
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
improved performance of coated rebar is due to excellent compatibility at the steel-concrete
interface. It can be concluded that application of Cement polymer anticorrosive coating
improves the bond strength development of rebar with concrete appreciably.
Table 1.Observation on Pull-out test for Rust free, Rusted and Coated rebars
Table 2. Bond Strength of Rust free, Rusted and Coated Rebars
Table 3 shows the ultimate bond strength values for Rust free, Rusted and Coated
rebars. It can be observed that the ultimate bond strength values for Rust free rebar is 4.5%,
13% more than the Rusted and Coated rebars respectively for 16mm diameter rebar. Incase of
20mm rebar there is a reduction in ultimated bond strength value for Rust free rebar of the
order 7% as compared to Rusted rebar where as coated rebar exhibits similar behaviour. It
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
can be concluded that presence of Cement polymer anticorrosive coating at the steel-concrete
interface did not influence any modifications in the Ultimate bond strength values.
Accoriding to IS 13620 – 1993, the reduction in bond strength due to the presence of coating
is permitted as 20%. Whereas Cement polymer anticorrosive coating improves the bond
strength appreciably of the order of 30% as compared to Rust free rebar.
Table 3. Ultimate Bond Strength of Rust free, Rusted and Coated rebars
CONCLUSIONS
Based on the tests conducted on 16mm, 20mm TMT rebar with Rusted, Acid pickled (Rust
free) and Coated rebars, the following conclusions were drawn:
1.
There is an appreciable influence on development of bond strength of steel with
concrete due to the presence of rust and coating at the interface.
2.
Coated rebars improve the usable bond strength to 30-35% as compared to Rust free
rebars due to excellent compatibility at the steel-concrete interface.
3.
Presence of rust at the interface influences in reduction/increase in usuable bond
strength depending on the nature and severiety of corrosion.
4.
There is no major variation in ultimate bond strength for tested rebars, irrespective of
the type when tested at the age of 28 days.
5.
Cement polymer anticorrosive coated bars satisfy the codal requirements of IS 13620
– 1993 with respect to bond strength.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 3 Issue 2 No3 – March 2012
REFERENCES
1.
Cusens A.R. and Yu Z. (1992), ‗Pullout Tests of Epoxy-Coated Reinforcement in
Concrete‘, Cement and Concrete Composites, Vol.14, pp.269-276.
2.
Han-Seung Lee, Takafumi Noguchi, Fuminori Tomosawa (2002), ‗Evaluation of the
Bond Properties Between Concrete and Reinforcement as a Function of the Degree of
Reinforcement Corrosion‘, Cement and Concrete Research, Vol. 32, pp.1313-1318.
3.
IS 1786-1985 Specification for High Strength Deformed Steel Bars and Wires for
Concrete Reinforcement, Bureau of Indian Standards.
4.
IS 2770-Part I-1967 Methods of Testing Bond in Reinforced Concrete – Part I-Pullout Test, Bureau of Indian Standards.
5.
IS 13620-1993, Fusion Bonded Epoxy Coated Reinforcing Bars -Specification,
Bureau of Indian Standards.
6.
Kayyali O.A. and Yeomans S.R. (1995), ‗Bond and Slip of Coated Reinforcement in
Concrete‘, Construction and Building Materials, Vol. 9, No. 4, pp.219-226.
7.
Obada Kayali, Stephen R. Yeomans (2000), ‗Bond of Ribbed Galvanized Reinforcing
Steel in Concrete‘, Cement and Concrete Composites, Vol. 22, pp.459-467.
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